Digitally modulated signals are used to transport high-speed data, video and voice on cable networks. The high-speed signals are subject to a variety of impairments that can seriously impact the quality and reliability of the services being provided. Unfortunately, measuring the signal level or looking at the display on a conventional spectrum analyzer isn't enough to fully troubleshoot problems or characterize the health of a digitally modulated signal.
Delivery of data services over cable television systems is typically compliant with a data-over-cable-service-interface-specifications (DOCSIS) standard. The content of the digital signal is typically modulated using quadrature amplitude modulation (QAM). Current cable QAM standards use conventional forward error correction (FEC) and interleaving techniques to transmit the data downstream. FEC is a system of error control for data transmission in which the receiving device detects and corrects fewer than a predetermined number or fraction of bits or symbols corrupted by transmission errors. FEC is accomplished by adding redundancy to the transmitted information using a predetermined algorithm. However, impairments that exceed the correction capability or the burst protection capacity of the interleaving will not be corrected, and the digital data will be corrupted. Accordingly, technicians need to be able to detect impairments above and below the threshold at which digital signals are corrupted to be able to detect current and potential problems.
As with all modulation schemes, QAM conveys data by changing some aspect of a carrier signal, or the carrier wave, which is usually a sinusoid, in response to a data signal. In the case of QAM, the amplitude of two waves, 90° out-of-phase with each other, i.e. in quadrature, are changed, e.g. modulated or keyed, to represent the data signal.
When transmitting two signals by modulating them with QAM, the transmitted signal will be of the form:s(t)=I(t)cos(2πf0t)+Q(t)sin(2πf0t)
where I(t) and Q(t) are the modulating signals and f0 is the carrier frequency.
At the receiver, the two modulating signals can be demodulated using a coherent demodulator, which multiplies the received signal separately with both a cosine and sine signal to produce the received estimates of I(t) and Q(t), respectively. Due to the orthogonality property of the carrier signals, it is possible to detect the modulating signals independently.
In the ideal case I(t) is demodulated by multiplying the transmitted signal with a cosine signal:
                                                        r              i                        ⁡                          (              t              )                                =                                    s              ⁡                              (                t                )                                      ⁢                          cos              ⁡                              (                                  2                  ⁢                  π                  ⁢                                                                          ⁢                                      f                    0                                    ⁢                  t                                )                                                                                  =                                                    I                ⁡                                  (                  t                  )                                            ⁢                              cos                ⁡                                  (                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                                          f                      0                                        ⁢                    t                                    )                                            ⁢                              cos                ⁡                                  (                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                                          f                      0                                        ⁢                    t                                    )                                                      +                                          Q                ⁡                                  (                  t                  )                                            ⁢                              sin                ⁡                                  (                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                                          f                      0                                        ⁢                    t                                    )                                            ⁢                              cos                ⁡                                  (                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                                          f                      0                                        ⁢                    t                                    )                                                                           
Using standard trigonometric identities:
                                                        r              i                        ⁡                          (              t              )                                =                                                    1                2                            ⁢                                                I                  ⁡                                      (                    t                    )                                                  ⁡                                  [                                      1                    +                                          cos                      ⁡                                              (                                                  4                          ⁢                          π                          ⁢                                                                                                          ⁢                                                      f                            0                                                    ⁢                          t                                                )                                                                              ]                                                      +                                          1                2                            ⁢                              Q                ⁡                                  (                  t                  )                                            ⁢                              sin                ⁡                                  (                                      4                    ⁢                    π                    ⁢                                                                                  ⁢                                          f                      0                                        ⁢                    t                                    )                                                                                                  =                                                    1                2                            ⁢                              I                ⁡                                  (                  t                  )                                                      +                                                            1                  2                                ⁡                                  [                                                                                    I                        ⁡                                                  (                          t                          )                                                                    ⁢                                              cos                        ⁡                                                  (                                                      4                            ⁢                            π                            ⁢                                                                                                                  ⁢                                                          f                              0                                                        ⁢                            t                                                    )                                                                                      +                                                                  Q                        ⁡                                                  (                          t                          )                                                                    ⁢                                              sin                        ⁡                                                  (                                                      4                            ⁢                            π                            ⁢                                                                                                                  ⁢                                                          f                              0                                                        ⁢                            t                                                    )                                                                                                      ]                                            ⁢                                                                                                           
Low-pass filtering ri(t) removes the high frequency terms, i.e. containing (4πf0t)), leaving only the I(t) term, unaffected by Q(t). Similarly, we may multiply s(t) by a sine wave and then low-pass filter to extract Q(t).
A constellation diagram is a representation of a signal modulated by a digital modulation scheme, such as quadrature amplitude modulation or phase-shift keying. The constellation diagram displays the signal as a two-dimensional scatter diagram in the complex plane at symbol sampling instants, i.e. the constellation diagram represents the possible symbols that may be selected by a given modulation scheme as points in the complex plane. Measured constellation diagrams can be used to recognize the type of interference and distortion in a signal.
By representing a transmitted symbol as a complex number and modulating a cosine and sine carrier signal with the real and imaginary parts, respectively, the symbol can be sent with two carriers, referred to as quadrature carriers, on the same frequency. A coherent detector is able to independently demodulate the two carriers. The principle of using two independently modulated carriers is the foundation of quadrature modulation.
As the symbols are represented as complex numbers, they can be visualized as points on the complex plane. The real and imaginary axes are often called the in phase, or I-axis and the quadrature, or Q-axis. Plotting several symbols in a scatter diagram produces the constellation diagram. The points on a constellation diagram are called constellation points, which are a set of modulation symbols that comprise the modulation alphabet.
In QAM, the constellation points are usually arranged in a square grid with equal vertical and horizontal spacing, although other configurations are possible. The most common forms are 16-QAM, 64-QAM, 128-QAM and 256-QAM. By moving to a higher-order constellation, it is possible to transmit more bits per symbol; however, if the mean energy of the constellation remains the same, in order to make a fair comparison, the points must be closer together and are thus more susceptible to noise and other corruption, i.e. a higher bit error rate. Accordingly, a higher-order QAM will deliver more data less reliably than a lower-order QAM. 64-QAM and 256-QAM are often used in digital cable television and cable modem applications. In the United States, 64-QAM and 256-QAM are the mandated modulation schemes for digital cable as standardised by the SCTE in the standard ANSI/SCTE 07 2000.
A typical QAM analyzer design includes a user interface, e.g. a keypad and a display, and possibly an Ethernet or other external connection for connection to a personal computer. A tuner is used to select a digitally modulated signal of interest, and a QAM demodulator provides several elements indicative of the received signal, such as carrier frequency acquisition, carrier phase tracking, symbol rate tracking, adaptive equalizer, and J.83 channel decoding. By probing into the elements of the QAM demodulator one can retrieve information on MER, pre- and post-FEC BER, and channel response, which is part of physical layer testing.
To troubleshoot a subscriber's premises with a signal problem, a technician will travel to the premises or a hub nearby, and conduct a variety of tests on the digitally modulated signal, e.g. RF level, MER, pre- and post-FEC BER, and an evaluation of the constellation for impairments. In addition, the technician may look at the equalizer graph for evidence of micro-reflections, and check in-channel frequency response and group delay. Moreover, if the QAM analyzer is able, the measurements are repeated in the upstream direction. Unfortunately, all of the test results require a certain degree of experience, knowledge and skill to interpret, potentially resulting in differing explanations for the problem depending on the technician. BER measurements require that the test instrument have the capability to fully decode the digital signal, and require long sampling time to detect low bit error rates. Furthermore, post-FEC BER shows only the impairments the exceed the correction capability of the FEC and interleaving
United States Patent Applications Nos. 2005/0144648 published Jun. 30, 2005 in the name of Gotwals et al; 2005/0281200 published Dec. 22, 2005 to Terreault; 2005/0286436 published Dec. 29, 2005 to Flask; and 2005/0286486 published Dec. 29, 2005 to Miller disclose conventional cable signal testing devices. U.S. Pat. Nos. 6,611,795 issued Aug. 26, 2003 to Cooper, and 7,032,159 issued Apr. 18, 2006 to Lusky et al, and United States Patent Application No. 2003/0179821 published Sep. 25, 2003 to Lusky et al relate to error correcting methods. PCT Patent Publication No. WO 2006/085275 published Aug. 17, 2006 to Moulsley et al relates to estimating the BER based on the sampled amplitude of the signal.
An object of the present invention is to overcome the shortcomings of the prior art by providing a fast and sensitive measurement of impairments on a QAM digital channel without requiring full decode capability of the QAM signal, and without requiring a vast amount of expertise.